Low intensity ultrasounds are widely used in medicine for diagnostic procedures, i.e. echography. For 10 years, high intensity ultra-sounds have shown to be an efficient means to induce tissue necrosis by hyperthermia for treatment procedures. Various therapeutic probes have been designed for minimally invasive therapeutic procedures and can be classified in two groups: external probes and internal probes.
External probes are designed to mimic the shape of the surface of the patient's body. Ultrasound transmitters are displayed in a concentric fashion to optimise the ultrasound waves focalization.
Internal/interstitial probes are inserted inside the body of the patient. There are three main categories: endo-cavity, endovascular or percutaneous probes.
A. Endocavity Probes
Endocavity probes are designed to be introduced in natural body holes such as the rectum, the vagina or the oesophagus. For example, US 2007/239,011 describes a medical probe for the delivery of high intensity focused ultra-sound (HIFU) energy to a patient's organ. Such a probe comprises a plane-shaped probe body inserted through a natural cavity of a patient, and a plurality of leaves to be applied to the surface of the organ, to deliver ultra-sound energy to the inside of the organ.
B. Endovascular Probes
Endovascular flexible probes are in development to treat cardiac atrial fibrillation or venous insufficiency.
C. Percutaneous Interstitial Probes
Percutaneous interstitial probes have initially received poor interest since they require a tissue penetration whereas previous probes don't penetrate the tissue. Nevertheless such percutaneous interstitial probes have been proposed for treating deep-seated tumours that cannot be reached with extra-corporeal, endocavity or endovascular high-intensity focused ultrasound probe. The ultrasound source is brought as close as possible to the target in order to minimize the effects of attenuation and phase aberration along the ultrasound pathway. Most-described ultrasound percutaneous probes are sideview emission probes whose active element is water-cooled and operates at a rather high frequency (above 3 MHz) in order to promote heating. Most described ultrasound percutaneous probes are not MRI compatible so that treatment monitoring is somewhat hazardous.
For clinicians, ultrasounds are a promising technology. To extend the applicability of ultra-sound therapy to a broad variety of medical treatments, there is a need to solve the following inconveniences:
In particular, external probes, although non intrusive, have shown consistent inconvenient: ultrasound attenuation, phase aberration and ultrasound defocalization by tissue structure (bone, tissue interfaces . . . ), targeting limits do to the constant body movement (respiratory, diaphragm . . . ), long treatment duration, unknown consequences on crossed normal tissue by the ultrasounds pathway, complexity of the probes with nowadays hundreds of ultrasound transducers, complexity to make the system MRI compatible and MRI adaptable.
In particular, sideview interstitial/internal probes require clinician manipulation of the probe during treatment such as a 360° rotation or a longitudinal translation to treat the whole lesion leading to a lack of precision and reproducibility.
In particular for all existing probes, none can perform histological characterisation or tissue biopsy, meaning that a biopsy procedure is necessary days before treatment. For all existing probes, none can perform a tissue resection after the thermal treatment. Indeed, hyperthermia treatment of a tumour will gender a serious tumour volume increase (mass effect) as shown in a previous clinical trial (Carpentier & al., “Real-time Magnetic Resonance-Guided Laser Thermal Therapy of Metastatic Brain Tumors”, Neurosurgery, 63 ONS Suppl 1:21-29, 2008). Such volume increase is most of the time incompatible with preservation of the normal surrounding tissue and can limit the development of such minimally invasive ultrasound therapy systems.